1. Field of the Invention
The present invention relates to an overflow device for a carburetor having a float chamber and a float provided in the float chamber wherein a constant fuel level in the float chamber is formed by the float.
2. Description of Background Art
In a float type carburetor mounted on an internal combustion engine, the amount of fuel supplied from a fuel tank and flowing into a float chamber is adjusted by a float valve operated in concert with vertical movements of a float floating in the fuel stored in the float chamber, thereby maintaining a fuel level in the float chamber at a constant level. As a result, a proper amount of fuel is drawn from a nozzle by a vacuum produced in an intake passage, and is mixed with air passing through the intake passage, thereby forming a fuel mixture having a proper air-fuel ratio.
However, if the float valve is not tightly closed because of foreign matter lodged between the float valve and a valve seat in such a float type carburetor, for example, the fuel is excessively supplied into the float chamber beyond the constant fuel level, so that a fuel mixture having a proper air-fuel ratio is not formed thus causing a poor operating condition for the internal combustion engine. Such an excess rise in fuel level is prevented by providing an overflow pipe having one end opening to a fuel level forming position higher than the constant fuel level by a given value and the other end communicating with the outside of the float chamber to discharge the excess fuel above the fuel level forming position through the overflow pipe to the outside of the float chamber.
Normally, the upper end opening of the overflow pipe is open above the constant fuel level in the float chamber. Accordingly, when fuel in the float chamber forms waves because of fluctuations of a vehicle body as of a motorcycle whose vehicle body fluctuates largely or because of vibrations of the internal combustion engine, a part of the fuel may rise up to the upper end opening of the overflow pipe directly or indirectly as by rebounding of the fuel on the wall of the float chamber and may flow out of the float chamber through the overflow pipe, causing environmental pollution or poor fuel economy in some cases. Various techniques have been proposed to suppress such undue emission of the fuel from the overflow pipe.
In an overflow device for a carburetor disclosed in Japanese Patent Laid-open No. 10-159655, for example, a cage type valve storing member is fixed to an upper end portion of an overflow pipe, and a vertically movable valve is inserted in the valve storing member so as to float on the surface of fuel in a float chamber. When the fuel level is lower than an overflow level, the valve comes into close contact with a fuel inlet of the overflow pipe to close the fuel inlet, whereas when the fuel level reaches the overflow level, the valve floats on the fuel surface to open the fuel inlet. Even when the fuel in the float chamber forms a wave, the fuel inlet of the overflow pipe is not opened so far as the valve does not float on the fuel surface. Accordingly, it is considered that fuel emission from the overflow pipe due to causing waves in the fuel may be suppressed.
However, the conventional overflow device disclosed in the above publication has a complicated and costly structure because the valve storing member and the valve are provided at the upper end portion of the overflow pipe. Further, the fuel inlet of the overflow pipe serving also as a valve seat for the valve is required to have a shape for making the fuel inlet to be tightly closed by the valve. Accordingly, it is difficult to apply this overflow device directly to a carburetor having an existing overflow pipe.
It is accordingly an object of the present invention to provide an overflow device for suppressing undue emission of fuel from an overflow pipe which can be produced with a simple structure at a low cost, and can be simply applied also to any existing carburetors.
In accordance with the present invention, there is provided in a carburetor having a float chamber and a float provided in said float chamber wherein the amount of fuel flowing into said float chamber is adjusted according to behavior of said float to thereby form a constant fuel level in said float chamber. An overflow device includes an overflow pipe having an upper end opening exposed to a space in said float chamber defined above said constant fuel level. A shield member is arranged above said constant fuel level and includes a side wall extending along an upper end portion of said overflow pipe between an upper position above said upper end opening and a lower position below said upper end opening so as to define a shielded space around said upper end opening. The shielded space is kept in communication with said space in said float chamber through a fuel opening for allowing the fuel to flow into and out of said shielded space and a vent opening is capable of suppressing the entry of the fuel into said shielded space. The fuel opening and the vent opening are formed by providing the shield member. The fuel opening is positioned only below the upper end opening. At least a part of the vent opening is positioned above the upper end opening.
According to the present invention, the shield member arranged above the constant fuel level has a side wall for defining the shielded space around the upper end opening. That is, the overflow device can be configured without any movable portions, i.e., with a simple structure at a low cost. Furthermore, the shield member can be simply applied to any existing overflow pipes. Even when the fuel in the float chamber causes a wave, the entry of the fuel from the vent opening positioned above the upper end opening of the overflow pipe into the shielded space can be suppressed, and the discharge of the fuel from the upper end opening can further be reduced owing to the presence of the shielded space. As a result, undue emission of the fuel through the overflow pipe to the outside of the float chamber can be suppressed.
Owing to the presence of the vent opening, the rise of the fuel level in the shielded space in the case of overflowing can be smoothly effected without a possibility that the pressure in the shielded space may be increased by the fuel flowing into the shielded space to hinder the rise of the fuel level in the shielded space. Accordingly, the rising speed of the fuel level inside the shielded space can be made substantially equal to that of the fuel level outside the shielded space. As a result, there is no possibility that the rising speed of the fuel level inside the shielded space may become lower than that of the fuel level outside the shielded space because of an increase in pressure inside the shielded space, so that it is possible to prevent an excess fuel from being supplied to the intake passage of the carburetor and to discharge the excess fuel through the overflow pipe to the outside of the float chamber at a preset overflow level.
In accordance with the present invention, the shield member further has a top wall positioned above the upper end opening and a bottom wall positioned below the upper end opening, the top wall and the bottom wall being contiguous to the side wall; the fuel opening is a hole formed through the bottom wall; and the vent opening is a hole formed through the side wall.
The shield member is a case member composed of the side wall, the top wall, and the bottom wall. Further, the fuel opening is a hole formed through the bottom wall, and the vent opening is a hole formed through the side wall. Accordingly, not only the entry of the fuel from the vent opening into the shielded space due to causing waves in the fuel, but also the entry of the fuel from the fuel opening into the shielded space due to causing waves in the fuel can be greatly suppressed.
In accordance with the present invention, the overflow device further includes an interfering member provided in the shielded space between the upper end opening and the vent opening in opposed relationship with the vent opening.
Even when the fuel enters the shielded space from the vent opening because of causing waves in the fuel level in the float chamber, the fuel having entered comes into collision with the interfering member interposed between the upper end opening of the overflow pipe and the vent opening so as to face the vent opening, so that the fuel having entered is hindered from advancing toward the upper end opening. Thus, the fuel reaching the upper end opening can be greatly reduced in amount, and the discharge of the fuel from the overflow pipe because of causing waves in the fuel level can therefore be further suppressed.
In accordance with the present invention, in the overflow device for the carburetor, the side wall is provided spirally about the upper end opening so as to extend along the upper end portion of the overflow pipe; and the vent opening is formed by an outer end of an outermost side wall portion positioned radially outermost of the side wall and an inner side wall portion positioned radially inside of the outermost side wall portion.
Even when the fuel enters the shielded space from the vent opening, most of the fuel having entered comes into collision with the inner wall surface of the outermost side wall portion and the outer wall surface of the inner side wall portion, and is therefore hindered from advancing towards the upper end opening of the overflow pipe. Accordingly, the fuel reaching the upper end opening can be greatly reduced in amount, and the discharge of the fuel from the overflow pipe because of causing waves in the fuel level can therefore be further suppressed.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.